Try using a #1 (0.228) tap drill and give the spiral flute another try. I use those almost exclusively in 304. Normally I tap 1/4-20, 5/16-18, and 1/2-20, never tapped a 1/4-28 in SS, so don't know how that would go. I ran about 11 hours worth of tapped parts in 304 yesterday with a spiral flute.